1. Field of the invention
The present invention concerns enameled wire with high resistance to partial discharges.
2. Description of the prior art
Insulating electrical wires with enamel provides a thin insulation on electrical wires. It can become insufficient for some applications. This is the case when such wires are used for the windings of alternating current motors, for example. Such motors are controlled by frequency converters that operate at higher and higher frequencies which can lead to premature aging of the enameled wires of the windings. Such aging is due in particular to partial discharges in the windings.
Document U.S. Pat. No. 4,537,804 describes a corona-resistant enameled wire which includes an electrical conductor and two layers of enamel on the latter. The two enamel layers are made from different resins. The resin of at least one of the two enamel layers contains very fine particles of alumina dispersed in the resin.
Charging an enameling resin with weakly conductive particles selected from metal oxides and organo-metallic compounds to improve the corona-resistance of the resin is known in itself.
Making a winding with contiguous turns using a wire insulated in the above manner necessarily requires that at least the outermost enamel layer be the one charged with weakly conductive particles to assure peripheral continuity of the exterior layers of the various turns and to distribute the electrical field across all of the turns.
An enameled wire of the above kind is flexible but less so than an enameled wire in which the resin of the enamel layers is not charged with such particles. It also has a smooth surface, but less so than the surface of a wire with an exterior enamel layer that is not charged. As a result winding an enameled wire of the above kind is more difficult and the performance obtained after winding is often not so good as was hoped for.
An aim of the present invention is to avoid the above problems.